Adding extra_specs to flavor format

We have added a new API extension FlavorWithExtraSpecs which when
enabled, will add the extra_specs of a flavor to the flavor
show and detail calls.
This handles the extra_specs field while parsing the flavor response
in the _format_flavor method.

Implements blueprint add-extra-specs-to-flavor-format

Change-Id: I46faaab69381c07b4c1466188a9c23014d869418
Merge "Adding extra_specs to flavor format"